Stretch wrapping utilises film having the property known as "memory", that is to say a tendency to recover its former shape or size, after having been strained beyond its elastic limit, some short while after the straining force has been relaxed.
Plastics wrapping film is normally provided to the user as a feed roll of unstretched film. Film is customarily drawn from the roll and applied to the object by securing an end of the film to the object, and then either rotating the object or orbiting the feed roll around the object. A number of types of pre-stretching mechanisms have been proposed hitherto whereby the web of film extending from the feed roll to the object is stretched as it is drawn from the roll before being applied to the object.
Such pre-stretching of the film is advantageous for a number of reasons, for example;
the pre-stretched film may be applied to the object under a relatively low lay-up tension, as the subsequent contraction of the film ensures a desirably tight wrap (this is particularly advantageous when an assemblage of relatively small or lightweight articles is to be wrapped, because it reduces the likelihood of the articles being displaced by the wrapping process), PA1 moderate or properly controlled pre-stretching improves the mechanical properties of the film, and PA1 it is economical, in that a given weight of film may be extended to provide effectively more wrapping material.
The simplest and least desirable prior known pre-stretching apparatus merely applies a brake to the roll of film. Such apparatus is disclosed, for example, in U.S. Pat. No. 3,867,806 (Lancaster) and U.S. Pat. No. 4,077,179 (Lancaster). In these and other instances wherein a brake is applied to the feed roll, the stretching is induced by using a high lay-up tension. This suffers from the disadvantages inherent in the use of a high lay-up tension indicated above. Furthermore, stretching occurs throughout the whole length of the flight of web extending from the feed roll to the object, so that the overall increase in length is large, and becomes unmanageable unless the rate of pre-stretch is limited to undesirably low values. Furthermore, stretching over a long length of film causes excessive contraction in the width direction of the film.
An improvement on simple brake devices is disclosed in Australian patent No. 536099 (Lancaster), wherein the web of film is trained around two rollers interposed between the feed roll and the object to be wrapped. Those rollers are connected by drive transmission means such that they necessarily rotate at different speeds or in different directions. This results in the stretching of the short length of web between the rollers.
Another prior art proposal is disclosed in Australian patent No. 589065 (Underhaug), wherein a braking roller in contact with the feed roll is connected to a stretching roller in contact with the drawn-off web by drive transmission means ensuring that the surface speed of the stretching roller is greater than that of the braking roller, so that a small and relatively constant length of web extending from the feed roll to the stretching roller is pre-stretched.
The braking and stretching rollers of the Underhaug device are mounted on a swinging lever whereby the web tension acting on the stretching roller is effective to load the braking roller against the feed roll. This ensures that braking contact between the braking roller and the feed roll is maintained as the feed roll diminishes in size. The lever introduces a mechanical advantage and ensures high pressure braking contact, indeed this is described as an advantage of the Underhaug device over its prior art.
In most respects the Underhaug device performs well when used in wrapping apparatus wherein the object to be wrapped rotates to draw a web of film from a positionally fixed feed roll. However it is not well adapted for use in wrapping apparatus (referred to hereinafter as "orbital wrapping apparatus) wherein the relative rotational motion between the object and the feed roll is effected by causing the feed roll to orbit around a positionally fixed object or part of an object, as exemplified in our Australian patent No. 653255.
In such orbital wrapping apparatus the feed roll is usually carried on an orbiting shuttle, which may be required to pass through the bore of an annular object, and which, for that reason, or merely to minimize the mass of the orbiting components, is kept as small as possible. The Underhaug pre-stretching mechanism, with its swinging lever, is not sufficiently compact for ready installation on a small shuttle. Furthermore, the orbital path of the shuttle is usually non-circular and inertial effects acting on the swing mounted stretching roller may affect the tension generated in the web.
The high braking efficiency of the Underhaug device, due to the high pressure between the braking roller and the feed roll arising from the mechanical advantage of the lever, while normally desirable, has been found to be a disadvantage under some circumstances, in particular when the shape of the object being wrapped and/or the shape of the orbital path are such that the rate of draw-off of web from the feed roll is erratic and subject to marked and sudden increases. In such instances the sudden increase in web tension needed to accelerate the feed roll causes a simultaneous severe application of the braking roller tending to prevent such acceleration. It has been found that, at best, this causes over stretching and, at worst, can cause the web to fail in tension and eventually snap.